2D Materials for K Ion Batteries in Different Electrolyte Systems
Potassium ion batteries are potential energy storage devices owing to their low cost and good K+ diffusion kinetics due to the small Stoke’s radius. Here, we report the potassium ion storage behavior in 2D materials including transition metal dichalcogenides and graphites . Their electrochemical behaviors in different electrolyte systems are explored. Meanwhile, their phase transitions during K ion intercalation at the atomic level are explored via High Resolution Transmission Electron Microscopy (HRTEM) and ex-situ X-ray diffraction (XRD) combined with Rietveld refinements. The layered TiS2 cathode material demonstrated outstanding potassium storage cycling and rate performances in ether-based electrolyte with a capacity of 80 mAh g-1 at 20 C and 63 mAh g-1 after 600 cycles (1.5 V-3.0 V, corresponding to 4.8 A g-1). In the TiS2, it undergoes a second-stage structure to be K0.11TiS2 and then first-stage K0.56TiS2. The K cations intercalate into trigonal prismatic sites with a sliding of Ti-S plane by 120 degree-rotation, rendering a √3×√3 commensurate superstructure along the  zone.